I have noticed a problem with patterns created in Illustrator CS6 that are dragged to Photoshop CS6. In Illustrator when patterns are created, sometimes a fine white line (hair line?) can be seen, but it disappears if you zoom in/out. It also does not show up with direct printing from Illustrator. However if I drag the pattern to Photoshop CS6 these lines will show up and also appear when printed.
If the pattern is exported form Illustrator and "Art Optimized Antialiasing" is chosen, then usually the lines will not appear in photoshop, but this is more time consuming than the typical drag and drop that I am used to. See the explanation here -